Radio communication systems in recent years employ many integrated circuits (ICs) for outputting an unbalanced signal. In such a radio communication system, an unbalanced signal output from an IC passes through a filter, and the unbalanced signal that has passed through the filter is converted into a balanced signal by an unbalanced to balanced converter (balun). Consequently, the number and size of passive parts used in the radio communication system are reduced, thereby reducing the size of and simplifying the radio communication system.
Attempts have heretofore been proposed to reduce the size of and simplify radio communication systems by (1) forming a filter and a balun integrally in a dielectric substrate, or (2) forming a λ/4 resonator and a λ/2 resonator integrally in a dielectric substrate, or (3) setting a frequency close to the frequency band used by the balun as an attenuation pole for efficiently attenuating signals outside of the passband (see Patent Document 1, for example).
According to another attempt to reduce the size of and simplify radio communication systems, a resonator is provided by a via hole formed in a dielectric substrate which comprises a plurality of laminated dielectric layers, the via hole extending in the laminated direction of the dielectric layers (see Patent Documents 2 through 6, for example).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-87008;
Patent Document 2: Japanese Laid-Open Patent Publication No. 7-142914;
Patent Document 3: Japanese Laid-Open Patent Publication No. 8-335803;
Patent Document 4: Japanese Laid-Open Patent Publication No. 9-35936;
Patent Document 5: Japanese Laid-Open Patent Publication No. 10-28006; and
Patent Document 6: Japanese Laid-Open Patent Publication No. 2000-165171.